1. Field of the Invention
The present invention relates generally to improved interbody spinal fusion implants for the immobilization of adjacent vertebral bodies and to a method for deployment thereof. In particular, the invention relates to interbody spinal fusion implants and methods for deployment thereof that significantly preserve the structural support of the dense endplate and subchondral bone regions of the adjacent vertebral bodies while also penetrating those endplates so as to access the vascular subchondral bone of those vertebral bodies for the purpose of achieving interbody spinal fusion at least in part through the implants themselves.
2. Description of the Prior Art
Surgical interbody spinal fusion refers to the method of achieving a bridge of bone tissue in continuity between adjacent vertebral bodies and across the disc space to thereby substantially eliminate relative motion between these adjacent vertebral bodies. The term xe2x80x9cdisc spacexe2x80x9d refers to the space between adjacent vertebral bodies normally occupied by a spinal disc. The spinal disc that normally resides between the adjacent vertebral bodies maintains the spacing between those vertebral bodies and, in a healthy spine, allows for the normal relative motion between the vertebral bodies.
Numerous implants to facilitate fusion have been described by Cloward, Brantigan, Michelson, and others, and are known to those skilled in the art. Such fusions have also been achieved with the use of bone grafts placed between the vertebral bodies, such as taught and practiced by Dr. Cloward. Generally, cylindrical implants, which may be threaded, offer the advantage of conforming to an easily prepared recipient bore spanning the disc space and penetrating into each of the adjacent vertebral bodies. Such a bore may be created by use of a drill. Drilling of the bore, however, removes a portion of the endplates and of the subchondral bone.
Human vertebral bodies have a hard outer shell of compacted, dense cancellous bone (sometimes referred to as the cortex) and a relatively softer, inner mass of cancellous bone. Just below the cortex adjacent the disc is a region of bone referred to herein as the xe2x80x9csubchondral zonexe2x80x9d. The outer shell of compact bone (the boney endplate) adjacent to the spinal disc and the underlying subchondral zone are together herein referred to as the boney xe2x80x9cend plate regionxe2x80x9d and, for the purposes of this application, is hereby so defined to avoid ambiguity. The endplate region constitutes the densest bone available to support the fusion implant over its length, and removal of this endplate region by the practice of creating a bore into the vertebral bodies results in the implant coming to rest on the softer and less dense cancellous bone that lies beneath the endplate deeper within the vertebral body.
Other spinal fusion implants are known that incorporate a modified cylindrical or a tapered cylindrical shape that also require the use of a drill to create a bore across the disc space and also result in the removal of a portion of the endplate. Inasmuch as the upper and lower vertebral bodies - contacting surfaces of these types of implants are arc-shaped, absent arching the recipient bed in the vertebral body by drilling, it would not be possible to gain the contact between the vertebral bodies and implant needed to achieve fusion. Such arching of the vertebral bodies to receive the implant results in the removal of the endplate.
Non-cylindrical implants that are pushed into the disc space after a discectomy are also known in the art. While these push-in implants do have the advantage of supporting the adjacent vertebral bodies by contacting a substantial portion of the vertebral endplates, they do not offer the advantages associated with threaded cylindrical implants that are screwed into a bore in the adjacent vertebral bodies to more securely hold these implants in their final fully seated positions. Further, unless the endplate is at least partially decorticated, i.e. worked upon to access the vascularity deep to the outer most aspect of the endplate itself, fusion will not occur.
Non-cylindrical spinal fusion implants that are inserted between the endplates of adjacent vertebral bodies and then rotated 90 degrees into place are also known. However, their cross-sectional configuration causes either unwanted over-distraction of the vertebral bodies as they are rotated or under-distraction between the adjacent vertebral bodies once rotated. For example, an implant having an approximately square or rectangular cross-section when rotated in either a clockwise or counterclockwise direction will result in a maximum distraction of the disc space when the diagonal of the implant is at a right angle (90 degrees) to the adjacent vertebral endplates. This amount of distraction is greater than that achieved by the implant when either of its opposed sides are in contact with the adjacent vertebral bodies. If the space between the adjacent vertebral bodies is too small or the amount of attempted distraction too great, rotation of the implant will either not be possible or the vertebral bodies will be broken. If the space between the adjacent vertebral bodies is sufficiently large to permit rotation of such an implant, then when the implant is rotated to its final position with its opposed sides in contact with the adjacent vertebral bodies, insufficient distraction will be achieved between the vertebral bodies as the opposed sides will have a lesser height between them than the diagonal which rotated through that same space. It should be noted that distraction within the elastic range of deformation is highly desirable because it secures the implant, allows the implant to stabilize the adjacent vertebral bodies relative to each other, and provides the most space for the neural elements both passing through and exiting through those vertebral segments.
Therefore, there exists a need for a spinal fusion implant that permits the endplate region of the adjacent vertebral bodies to be substantially preserved while nevertheless accessing the underlying bone vascularity and which implant can be rotated 90 degrees within the disc space to achieve the optimal distraction in the range of elastic deformation and short of plastic deformation and tissue failure.
The present invention is an interbody spinal fusion implant allowing for the growth of bone from vertebral body to vertebral body through the implant. The present implant is designed to be pressed into a disc space in which the adjacent vertebral endplate regions have been substantially preserved. That is not to say that the endplates must be pristine. Rather, the implant as contemplated in the preferred embodiment described herein is designed to be used in a disc space where a structurally significant amount of the endplate subchondral region remains. A preferred implant of the present invention is deployed by rotating it 90 degrees about its long axis such that a body portion of the implant contacts and supports the adjacent vertebral endplate regions while projecting members, for example fins or blade-like projections, are driven and then extend into the deeper interior bone of those vertebral bodies.
It is an object of the present invention to provide an improved spinal fusion implant configured to permit vertebral body to vertebral body fusion through the implant. The implant is inserted between adjacent vertebral bodies and then rotated 90 degrees into place without over-distracting the vertebral bodies apart while penetrating the vertebral endplates to access the underlying bone vascularity and to lock the implant into position, thereby stabilizing the adjacent vertebral bodies relative to the implant and relative to each other. The phrase xe2x80x9cwithout over-distractionxe2x80x9d is defined as distracting the vertebral bodies in the range of elastic deformation and short of plastic deformation and tissue failure. To avoid any ambiguity regarding the phrase xe2x80x9cwithout over-distraction,xe2x80x9d this phrase and the individual words contained therein are not being used as they may be in their normal or ordinary use, but are being used as defined in this application only.
It is a further object of the present invention to provide an improved interbody spinal fusion implant that may, but need not necessarily, be inserted without the need to drill a bore across the disc space and into the adjacent vertebral bodies, thereby substantially preserving the endplate regions of the adjacent vertebral bodies while still providing access to the subchondral vascular bone vital to interbody fusion.
Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be evident from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
The following description is intended to be representative only and not limiting and many variations can be anticipated according to these teachings and are included within the scope of this inventive teaching.
To achieve the foregoing objects, and in accordance with the present invention, as embodied and broadly described herein, there is provided an improved interbody spinal fusion implant for insertion across a disc space between adjacent vertebral bodies of a human spine. In a first embodiment, the implant has a body having an insertion end, side walls, upper and lower walls, and a cross-section wherein the side walls intersect the upper and lower walls at two diametrically opposed corners and two diametrically opposed arcs. The implant also has one or more fin-like protrusions extending outwardly from the upper and lower walls so that when the implant is rotated approximately 90 degrees into its final position between the vertebral bodies, the protruding fins penetrate the endplates of the adjacent vertebral bodies. The implant can be configured so as to have only a single direction of rotation or to be symmetrically rotatable. When the implant is configured so as to be rotatable in either direction about its long axis, i.e. it is symmetrical, then the junctions of the side walls to the upper and lower walls will preferably each be arced. As used herein, the term xe2x80x9cside wallsxe2x80x9d refers to those portions of the implant that extend between the adjacent vertebral bodies after the implant has been rotated into its final position within the disc space. The xe2x80x9cupperxe2x80x9d and xe2x80x9clowerxe2x80x9d walls refer to those portions of the implant that contact the vertebral bodies cephalad and caudad, respectively, after the implant is rotated into its final position within the disc space and which surfaces bear the vertebral bodies penetrating fin-like projections.
There are numerous claimed variations on the above-described implant. By way of example only, the side walls and upper and lower walls, respectively, may be generally parallel to one another. The side walls may physically contact the adjacent vertebral bodies upon initial insertion between the vertebral bodies before the implant is rotated into its final position. The side and/or upper and lower walls may be configured with openings to allow bone to grow therethrough, and the implant may have a hollow portion that can be loaded with a fusion-promoting material to promote fusion between the adjacent vertebral bodies. The upper and lower walls may be angled in various directions to one another to account for lordosis in the spine, and/or may be contoured to match the natural contours of the endplates of the adjacent vertebral bodies.
The implant can be made of any material appropriate for human implantation within the spine and of sufficient strength to work for the intended purpose. Such materials include, but are not limited to, cortical bone, bone composite, plastics, carbon-fiber or other composites, ceramics, surgical grade implant quality metals such as titanium and titanium alloys, tantulum, and chomemoly alloy. The implant may further comprise of bioresorbable material and of materials that are bioactive or induce the production of bone vital for fusion. Such materials may be within the material of the implants, contained within the structure of the implant, or be a coating or treatment to the implant. Such materials include, but are not limited to, bone morphogenetic proteins, genetic factors, (genetic material coding for the production of bone) and converting factors to stimulate the formation, recruitment, and/or activity of osteoblasts, or other cells or cellular mechanism for bone production.
The diametrically opposed junctions are preferably arcuate, and more preferably arcs that can be configured in different ways. For example, the arcs may be arcs of radii and may further be each of the same radius. Or, the arcs may each be chords of the same circle, or quadrants of a circle. Likewise, the other of the diametrically opposed junctions may be corners, such as can form right angles. The other opposed junctions, alternatively, can be relieved, chamfered, or radiused as when it is desired to have an implant that can be rotated in either direction about its longitudinal axis.
The fins also may have a number of different configurations. Alternative fin configurations include protrusions having different heights, equal height, or varying lengths along a portion of the length of the implant as measured from either a central longitudinal axis passing through the implant or the upper and lower surfaces of the implant body from which the fins project. The fins or protrusions may also be of varied or constant thickness, or varied or constant spacing from fin to fin. The fin or protrusion may have a sharp leading edge and/or outer surface to facilitate cutting into the vertebral endplate region upon rotation of the implant and in a preferred configuration go from a knife-like, ramped, thin, and sharpened leading edge to a thickened and blunt trailing end. To avoid any ambiguity regarding what is intended as a body having upper and lower surfaces and fins projecting therefrom as used herein, an implant even if so formed so as to obscure the distinction between the upper and lower surfaces and projecting fins would nevertheless be within the scope of the terms and claims of the present application. It is understood that in this case there is still an area between the fins that would come to lie in support of each of the vertebral bodies at the surfaces adjacent the disc space, and such an implant is within the scope of the present invention.
The implant of the present invention need not be used alone. Rather, the implant can be used with a complementary implant to provide additional stability and fusion promotion. This complementary implant is novel in and of itself and comprises an alternative embodiment of the present invention. For example, a second implant of the type previously described can be rotated into place after the first implant, either in the same direction (i.e. clockwise or counterclockwise) as the first implant or preferably in an opposite direction from that of the first implant. The direction of rotation of the implant depends upon the location of the diametrically opposed junctions that are preferably arcs of radii. Those implants of a preferred embodiment having a single direction of rotation configuration, rotate in the direction that causes the arcs of radii, rather than the corners, to engage the endplates when the implant has been rotated approximately 45 degrees, so as to utilize the geometrical configuration of the arcs of radii to avoid over-distraction.
Once two such implants have been inserted into a disc space intermediate adjacent vertebral bodies, there may be room to insert a third specialized implant between those two but insufficient room to allow for the rotation of that third implant for its seating. Therefore, a preferred novel, complementary third implant has in its preferred embodiment ratchetings on its upper and lower surfaces to engage the endplates of the adjacent vertebral bodies and thereby gain stability within the disc space, and preferably may also have ratchetings on its sides to mate with similarly spaced ratchetings on the side walls of the first and second implants so as to be locked into place by each of those implants, which themselves are locked into the adjacent vertebral bodies. It is anticipated that the implants can interdigitate in other ways for similar purpose. The third implant may force the first and second implants further apart thereby trapping itself in place and enhancing the stability of the other two implants as well as the adjacent vertebral bodies relative to the implants and to each other. While ratchetings are preferred, other surfaces such as knurling or other structures to mechanically interdigitate the implant to the adjacent vertebral bodies and to the adjacent implants are included within the scope of the present teaching.
The present invention also includes a method for deploying at least one of the subject interbody spinal fusion implants across a disc space and into adjacent vertebral bodies within a human spine. The method comprises the steps of: removing at least a portion of the disc from between the adjacent vertebral bodies from vertebral endplate to adjacent vertebral endplate, and to a depth at least as great and preferably greater than the length of the implant, and to a width at least as great as the height of the implant as measured from fin tip to opposed fin tip where maximum for that implant; providing a first implant having an insertion end, a trailing end, side walls and upper and lower walls bearing protrusions, which protrusions are preferably, but not necessarily, in the form of fins extending outwardly from the opposed upper and lower walls. Preferably, the upper and lower walls have at least one, or alternatively a plurality of, openings passing therethrough so as to allow for the growth of bone in continuity from one of the adjacent vertebral bodies to the other of the adjacent vertebral bodies through the spinal fusion implant. The implant includes a cross-section with the side walls intersecting with the upper and lower walls at junctions, which preferably are two diametrically opposed arcuate portions. The method also includes the steps of inserting the implant by linearly advancing it between the adjacent vertebral bodies with the side walls facing the endplates of the adjacent vertebral bodies, and then rotating the implant 90 degrees about its long axis so that the surface projections extending from the upper and lower walls are driven into the bone of the adjacent vertebral bodies into a deployed position such that the fins are driven and penetrate the end plates of the adjacent vertebral bodies. When the implant is deployed, the upper and lower walls from which the fins extend will then be placed into contact and support through the endplate regions the adjacent vertebral bodies.
Another embodiment of the present invention includes the steps of removing disc material as described; attaching the implant to a hand-held driver instrument; retracting any bodily tissues including, but not limited to, neurological structures, vascular structures, and bodily organs to provide clear access to the space created; attaching to the implant a hand-held insertion instrument capable of engaging the implant to provide for both linear advancement and rotation; inserting the implant by linearly advancing the implant in the space created between the adjacent vertebral bodies with the side of the implant adjacent the vertebral endplates to a depth sufficient so the implant does not protrude from the spine; rotating the implant by use of an instrument and preferably the insertion instrument so that the preferred junctions of opposed arcuate portions contact the adjacent vertebral bodies; continuing to rotate the implant so that the fin-like projections of the upper and lower walls are driven through the adjacent surfaces of the adjacent vertebral bodies until the implant rotates approximately 90 degrees; and disengaging the insertion and/or rotation tool without derotation of the implant.
The method preferably includes the steps of providing a second implant having an insertion end, a trailing end, side walls and upper and lower walls with outwardly extending fins, at least the upper and lower walls having openings which pass therethrough that are sufficiently sized and configured to allow for the growth of bone in continuity therethrough from vertebral body to vertebral body in a structurally meaningful way so as to significantly bear load from vertebral body to vertebral body, the second implant having a cross-section with the side walls intersecting the upper and lower walls at junctions, which preferably are two diametrically opposed arcuate portions; inserting the second implant between the adjacent vertebral bodies with the side walls directed toward the adjacent vertebral bodies; and then rotating the second implant 90 degrees into a deployed position such that the upper and lower walls then contact and support each of the adjacent vertebral endplate regions while the fins, extending from the upper and lower walls, are then penetrably driven through the vertebral endplates.
As a substep of that method, the first implant may be deployed by rotating it 90 degrees in a first direction while the second implant may be deployed by rotating it in either the same direction or preferably in the opposite direction.
The method may further comprise lateralizing (more lateral) the first and second implant to provide a space between the first and second implants. The method may still further comprise placing within that space a third implant different in structure from the first and second implants in that while it is designed to be inserted by linear advancement, it is not designed to be rotated into place. The specialized third implant may include protrusions (ratchetings) on its outer walls so as to engage the implant to the adjacent vertebral bodies and to engage the third implant to the first and second implants. This specialized third implant preferably has upper and lower walls for contacting each of the adjacent vertebral bodies. The upper and lower walls have at least one opening to allow for the growth of bone in a mechanically meaningful way in continuity from a first adjacent vertebral bodies through the implant to the second of adjacent vertebral bodies. Further, a substep preferably for use when the implant is made of a material such as cortical bone, carbon fiber or any material less strong than titanium alloy, includes the use of a rotary broach or tap to provide slits in the vertebral endplates through which the fins are guided.
When the first and second implants rotate in opposite directions away from each other, the fins cut a path through the surfaces of the adjacent vertebral bodies longer than that occupied by the implant itself when deployed. This facilitates the implants being slid apart without tipping over, twisting,,or moving forward or back as the fins slide sideways relative to their long axis from more central to more lateral. In a preferred method, the present disc material including portions of the very strong annulus fibrosus resist such implant lateralization and tend to urge the first and second implants back centrally. Thus, the third implant is wedging apart the other two implants and in this situation obtains for itself and provides to the other implants and to the adjacent vertebral bodies an extra measure of stability.
It is to be understood that both the foregoing general descriptions and the following detailed description are exemplary and explanatory only and are not restrictive of the scope of the invention, which scope is defined solely by the appended claims.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention.